More memory from chemistry

As creating smaller and smaller circuits by traditional techniques is becoming …

As computers and components scale down further and further, traditional
manufacturing materials and methods become harder and harder to work
with. Current DRAM components have wires that are about 140nm apart from one
another, but a semi-conductor industry roadmap says that this spacing
must be shrunk to a mere 33 nm by the year 2020. This poses a huge
problem, as traditional lithographic techniques have
difficulty resolving features in that size range. To sidestep this
problem, researchers from around the globe are looking at novel
materials and methods.

Last week Ars highlighted a research report that discussed the
possibility of using a derivative of Prussian blue as a switching
material—a material whose molecular configuration can be easily
changed between multiple states. However this line of research is in
its infancy, and the material discussed needed to be kept at -150o C to
function properly. As one respondent pointed out, operating at room
temperature would format all your memory. Today Naturehas a letter
detailing research by a group of chemists and chemical engineers who
created a complete circuit that lies between the the switching material
discussed previously and traditional circuitry currently in use in
today's components.

The collaborative work by researchers from CalTech and UCLA created a
grid of nanowires—silicon wires on top running in one direction,
titanium wires on the bottom running perpendicular—with a
monolayer of switchable molecules, rotaxanes, sandwiched between them.
It is the shape of these
molecules that allowed for some interesting science: rotaxanes consist
of a linear central rod portion that is threaded through a molecular
ring that is free to move up and down the rod. At each end were bulky
molecular "caps" that kept the ring on the rod. On the outside edges of
these caps were specialized functional groups that allowed one end to
bind to the titanium wire, and one end to bind to the silicon wire.
Voltages applied to the wires would change the oxidation state of the
molecule by moving the ring to one end or the other, and thus change
the conductivity of the junction allowing it to register as a one or a
zero.

Contrary to the work reported last week, where an actual device was some
time off, the teams here created a functional device! The device has
a bit density of 160,000 bits in an area the size of a human white
blood cell, which corresponds to around 1011 bits/cm2 (a factor of 40
greater than current devices). Even though this is a big step forward,
the device was far from perfect. While the researchers did not test all
160,000 possible junctions, they tested a subsection and found that
only around 25 percent of the junctions were able to be classified as
"good" bits. Also, after a small number of switches, around 10, none of the
junctions were still able to be classified as "good".

Nevertheless, the team states that the high level of initially bad bits
is not an inherent show stopper. Given the 'cross-bar' geometry, and
lessons learned from operating the Teramac supercomputer (which had a
similar layout) they feel that hardware and software could be developed
to work around these bad bits—in a similar fashion to bad sectors
on a hard drive. The short life span of the devices is a bigger
immediate concern, one which they felt can be overcome with further
work, both on the design and manufacturing side of the device. However,
they have produced a functioning device with molecular switches, which
is a big step to achieving future computing goals.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.